With the recent increasing use of various electrical facilities and electronic devices, there arises a rapid increase in electromagnetic interference (EMI). It is pointed out that EMI not only causes mechanical errors and disturbances in electrical and electronic devices but also exerts adverse effects on the health of workers operating these devices. Thus, it has been required to control the intensity of electromagnetic wave generated from electrical/electronic devices not to exceed a standard or regulation level.
As a countermeasure against the above-described EMI, it is needed to shield electromagnetic wave, which can be self-evidently established by using the characteristics of a metal of not allowing electromagnetic transmission. Thus, there have been adopted, for example, a method of using a housing made of a metal or a highly conductive material, a method of inserting a metal plate between circuit baseboards and a method of coating a cable with a metal foil. However, a display of a CRT, a PDP or the like should be transparent, since an operator should recognize characters and so on indicated on the screen. However, the front face of a display frequently becomes opaque in each of the methods as cited above, which makes these methods unsuitable as an electromagnetic wave shielding method.
Compared with a CRT, etc., a PDP generates a particularly large amount of electromagnetic wave. Therefore, it is required that a PDP has stronger electromagnetic wave shielding properties. Electromagnetic wave shielding properties can be briefly expressed in surface resistivity. A light transmitting electromagnetic wave shielding material for CRT should have a surface resistivity of about 300 Ω/sq or less, while a light transmitting electromagnetic wave shielding material for PDP should have a surface resistivity of about 2.5 Ω/sq or less. Furthermore, a light transmitting electromagnetic wave shielding material for plasma TV for consumer use should have an extremely high conductivity, i.e., a surface resistivity of about 1.5 Ω/sq or less, preferably 0.1 Ω/sq or less.
Concerning transparency, a light transmitting electromagnetic wave shielding material for CRT should have a transparency of about 70% or more, while a light transmitting electromagnetic wave shielding material for PDP should have a still higher transparency of 80% or more.
To overcome the problems as discussed above, there have been proposed various materials and methods for achieving both of favorable electromagnetic wave shielding properties and high transparency with the use of a metal mesh having apertures, as will be described herein below.
(1) Conductive Fiber
For example, Patent Document 1 discloses an electromagnetic wave shielding material made of conductive fibers. However, this shielding material suffers from a problem that it has a broad mesh line width and, therefore, makes the screen dark when employed in shielding a display screen. As a result, characters indicated on the display become hardly visible.
(2) Mesh Obtained by Electroless Plating Process
There has been proposed a method which comprises printing a lattice pattern by using an electroless plating catalyst and then conducting electroless plating (see, for example, Patent Document 2, Patent Document 3 and so on). In this case, however, the printed catalyst has a broad line width of about 60 μm, which makes it unsuitable for a display with a need for a relatively narrow line width and a precise pattern.
Moreover, there has been proposed a method which comprises coating a photoresist containing an electroless plating catalyst, exposing and developing the same to form an electroless plating catalyst pattern and then conducting electroless plating (see, for example, Patent Document 4). However, a conductive film has a visible light transmittance of 72% and, therefore, only an insufficient transparency can be obtained thereby. Moreover, this method suffers from a problem in the production cost, i.e., highly expensive palladium should be employed as the electroless plating catalyst for removing most of the exposed part after the exposure.
(3) Mesh Obtained by Etching Process Using Photolithographic Technique
There has been proposed a method wherein a thin metal mesh film is formed on a transparent substrate by etching process using a photolithographic technique (see, for example, Patent Documents 5 to 8 and so on). Because of enabling fine processing, this method is advantageous in forming a mesh having a high aperture ratio (a high transmittance) and being capable of shielding even strong electromagnetic release. On the other hand, it suffers from a problem that the production process is troublesome and complicated and, therefore, costs high. Further, it is known that because of using the etching procedure, the lattice pattern has a problem that the intersecting points have broader line width than straight line parts. Furthermore, it has been pointed out that this method suffers from the problem of moire that should be overcome.
(4) Mesh Obtained by Copper-Plating on Conductive Metallic Silver and Development Sliver Using Silver Halide
There has been proposed a method of forming a conductive mesh with the use of conductive metallic silver obtained by developing a silver halide or a method of forming a conductive mesh by plating a mesh-like developed silver obtained by developing a silver halide with metallic copper (see, for example, Patent Documents 9 and 10).
In order to prevent malfunctions in, for example, remote controls, near infrared ray-cutting function is considered as an important factor required for the electromagnetic wave shielding films and PDP as described above, as reported in Patent Document 11 and Patent Document 12.
Patent Document 1: JP-A-5-327274
Patent Document 2: JP-A-11-170420
Patent Document 3: JP-A-5-283889
Patent Document 4: JP-A-11-170421
Patent Document 5: JP-A-2003-46293
Patent Document 6: JP-A-2003-23290
Patent Document 7: JP-A-5-16281
Patent Document 8: JP-A-10-338848
Patent Document 9: JP-A-2004-207001
Patent Document 10: JP-A-2004-221564
Patent Document 11: JP-A-9-247581
Patent Document 12: JP-A-10-75087